1. Field of the Invention
This invention relates to a lock-up control device for a lock-up torque converter of an automatic transmission.
2. Description of the Prior Art
In an automatic transmission using a torque converter of a vehicle, the torque converter is comprised of a lock-up clutch that completely locks up the torque converter while the vehicle is running at a high speed, and keeps the lock-up clutch in a lock-up clutch slip state, in which the lock-up clutch is engaged in such a manner as to slip slightly, while the vehicle is running at a predetermined intermediate speed in order to improve the fuel economy performance and reduce the vibrations.
The lock-up slip state assumes that the vehicle is running while an accelerator pedal is not so deeply depressed. If the accelerator pedal is deeply depressed in the lock-up clutch slip state contrary to the assumption, the engine speed considerably rises to rapidly increase the slip amount of the torque converter and raise the temperature of a facing in the engaged lock-up clutch. If no measures are taken to solve this problem, the durability of the lock-up clutch is deteriorated.
It is therefore an object of the present invention to provide a lock-up control device for a torque converter of an automatic transmission, which prevents deterioration of the durability of a lock-up clutch even if an accelerator pedal is deeply depressed.
To accomplish this object, the present invention provides a lock-up control device for a torque converter having a hydraulic lock-up clutch provided with a facing, comprising a slip corresponding amount detecting means for detecting a slip corresponding amount of the torque converter; and a lock-up slip termination means that terminates lock-up clutch slip control when the slip corresponding amount exceeds a first predetermined value during the slip lock-up control of the lock-up clutch. This prevents the temperature of the facing from rising due to the increase in slip amount of the lock-up clutch, and thus improves the durability of the lock-up clutch.
The slip corresponding amount may be the speed of change in the angle of throttle valve (throttle opening), the slip amount based on a difference between the engine speed and the turbine rotational speed, or engine torque.
In one preferred form of the present invention, the lock-up control device comprises a returning means for returning to the lock-up clutch slip control when the slip corresponding amount becomes smaller than a second predetermined value after termination of the lock-up clutch slip control. This enables reversion to the lock-up clutch slip control in early timing, and maintains the excellent fuel economy performance.
In a further preferred form of the present invention, the first predetermined value of the slip corresponding amount is determined to be greater at low oil temperatures than at higher oil temperatures. This enables continuation of the lock-up clutch slip control for a long period of time, and maintains the excellent fuel economy performance.